Both punches in a punch attack are affected by power unlike barrage which only buffs the first attack, so for example if you have an power stone and use tough fury you will have +4 attack which is 6 damage multiplied by 2, with the disadvantage of being affected by defence twice as much as normal. If you use fury then 2 punch attacks you will only spend 2 sp and deal 24 damage compared to 6 sp and 24 damage if you used 3 tough evil impacts.

Right now this method isn't useful against enemies with defence but as the game progresses we will get more infinity stones and each one you put into power will increase the punch damage by 2 and after enough power stones, fury double punch will actually be viable against upcoming enemies that have lots of hp but low or no defence especially because it costs only 2 sp for fury unless vetex decides to add a stronger attack against low defence enemies

Also the AS discord in the development updates it is shown that vetex has a second power stone meaning that we may get another magical stone next update.